CHARACTERIZATION OF AIR-ANNEALED, PULSED-LASER DEPOSITED ZNO-WS2 SOLID FILM LUBRICANTS BY TRANSMISSION ELECTRON-MICROSCOPY

ZnO-WS2 is a candidate high temperature solid film lubricant for aeros pace applications that exhibits adaptive lubricant behavior. In the as -deposited state, room temperature (RT) pulsed laser deposited (PLD) Z nO-WS2 films are amorphous, but when wear-tested, the crystalline phas es WS2, WO3 and ZnWO4 are produced. Of these, WS2 is a lubricant phase at low temperatures (less than or equal to similar to 450 degrees C) while ZnWO4, which is formed by reaction of the film with air, becomes lubricious above 600 degrees C. If this material is to be used at ele vated temperatures, the characterization of the microstructural and ch emical changes that occur when these films are heated in air is extrem ely important to the understanding of the dynamics of this system. As- deposited films and films heated in air at increasing temperatures to 800 degrees C were examined by transmission electron microscopy (TEM), Raman spectroscopy, and scanning electron microscopy (SEM). Cross-sec tional TEM (XTEM) of the as-deposited RT-PLD ZnO-WS2 films showed that they were fully dense and amorphous. A periodic structure was seen th at was due to density variations and was attributed to a smaller angul ar distribution of W in the plume compared to the other elements. At 5 00 degrees C, an approximately 37 nm-thick film of WO3 and ZnWO4 forme d at the surface. At 600 degrees C, a 150-200 nm-thick mixed oxide lay er of ZnWO4 and WO4 formed at the surface with the WS2 phase forming b elow it. The volume fraction of WS2 decreased with increasing depth fr om the surface. Above 600 degrees C, surface roughening of the film wa s seen as well as significant grain growth of the WO3 and ZnWO4 phases . The ZnO phase was not detected in any of the films heated in air. Th e dynamics of the nucleation of these lubricant phases are advantageou s with respect to applications: the high temperature lubricant phase, ZnWO4, is available at the surface while the low temperature phase, WS 2, remains intact to provide lubrication when the temperature is lower ed. (C) 1997 Elsevier Science S.A.